This is actually a rather similar dilemma to the problem of evil in Christian philosophy. The problem of evil is the question why an omni-benevolent, omniscient, and omnipotent God would permit bad things to happen. There's a lot of literature on this, and you could take cues from how the Christian God is able to get around this conundrum.

The easiest way for you (not available for most Christian philosophers) is to simply knock out one of the three legs of the tripod. If you're god isn't omniscient, then you arrange (with the help of the duplicity god) to always fall into the noble god's blindspots. If they're not omnipotent, you arrange things so that the noble god is either powerless or constrained in reacting. (For example, if there's some sort of rules of engagement between the noble god and the duplicity god, where the impostor can walk a fine line in actions which prevent the noble god from responding.) "Omni-benevolent" isn't applicable in this case, but the equivalent here is omni-caring. If, for some reason, the noble god doesn't really care about the impostor, then the impostor's rivals may find it difficult to get the noble god's help.

Even if these approaches don't work, there are other possibilities. One is the "greater good" invocation. The noble god deliberately allows the impostor to function because the small amount of deception now will allow a much larger (and better) amount of nobility later. For example, if the church is fossilized and most worshipers are apathetic, having an impostor rise through the ranks and eventually cause a great upheaval might revitalize the church. As far as the noble god is concerned, the impostor will get his comeuppance, but can be used as a valuable tool before then - to the extent the noble god may actively cover for the impostor.

There's also the "free will" argument. With the problem of evil, it can be argued that it can't really be "free" will if people don't have the ability to choose evil. (Or that actively rejecting evil is an important process in the development of souls.) In your case, since the duplicity god isn't necessarily rejecting worship of the noble god, the noble god could see the impostor as a means of "separating the wheat from the chaff". The antagonism of the impostor provides a means for testing believers and either strengthening their beliefs, or to figure out who actually believes and who's just going through the motions.

The key issue is deciding how you wish to limit your noble god. An "all powerful" god is a bit of a story killer (there's a reason for "deus" in deus ex machina). Your god needs to have limits, and those limits are what your impostor would exploit. These can be limits fundamental to the god's existence (like not being omnipotent or omniscient), or they can be self-imposed limits (like a divine contract with the the duplicity god, or allowing A to happen now to enable B to happen later).

This is actually a rather similar dilemma to the problem of evil in Christian philosophy. The problem of evil is the question why an omni-benevolent, omniscient, and omnipotent God would permit bad things to happen. There's a lot of literature on this, and you could take cues from how the Christian God is able to get around this conundrum.

The easiest way for you (not available for most Christian philosophers) is to simply knock out one of the three legs of the tripod. If your god isn't omniscient, then you arrange (with the help of the duplicity god) to always fall into the noble god's blindspots. If they're not omnipotent, you arrange things so that the noble god is either powerless or constrained in reacting. (For example, if there's some sort of rules of engagement between the noble god and the duplicity god, where the impostor can walk a fine line in actions which prevent the noble god from responding.) "Omni-benevolent" isn't applicable in this case, but the equivalent here is omni-caring. If, for some reason, the noble god doesn't really care about the impostor, then the impostor's rivals may find it difficult to get the noble god's help.

Even if these approaches don't work, there are other possibilities. One is the "greater good" invocation. The noble god deliberately allows the impostor to function because the small amount of deception now will allow a much larger (and better) amount of nobility later. For example, if the church is fossilized and most worshipers are apathetic, having an impostor rise through the ranks and eventually cause a great upheaval might revitalize the church. As far as the noble god is concerned, the impostor will get his comeuppance, but can be used as a valuable tool before then - to the extent the noble god may actively cover for the impostor.

There's also the "free will" argument. With the problem of evil, it can be argued that it can't really be "free" will if people don't have the ability to choose evil. (Or that actively rejecting evil is an important process in the development of souls.) In your case, since the duplicity god isn't necessarily rejecting worship of the noble god, the noble god could see the impostor as a means of "separating the wheat from the chaff". The antagonism of the impostor provides a means for testing believers and either strengthening their beliefs, or to figure out who actually believes and who's just going through the motions.

The key issue is deciding how you wish to limit your noble god. An "all powerful" god is a bit of a story killer (there's a reason for "deus" in deus ex machina). Your god needs to have limits, and those limits are what your impostor would exploit. These can be limits fundamental to the god's existence (like not being omnipotent or omniscient), or they can be self-imposed limits (like a divine contract with the the duplicity god, or allowing A to happen now to enable B to happen later).