Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as 101,325 Pa (1,013.25 hPa), which is equivalent to 1013.25 millibars,1 760 mm Hg, 29.9212 inches Hg, or 14.696 psi.2 The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth; that is, the Earth's atmospheric pressure at sea level is approximately 1 atm.
If 1013.25 millibars equal 760 mmHg, 1 mmHG equals 1.3332236 millibars. So 1,000 millibars, or 1 bar, equals 750.06173 mmhg. The question states that at 5,000 meters altitude the pressure is 2 bar, which equals 1500.1234 mmHg. Vesper's comment says that if the pressure is 2 bar at 5,000 meters it will be about 3.8 bar at sea level. Slarty's comment suggests the pressure should be about 3.2 bar at sea level.
3.2 bar would be 2,400.1975 mmHg, and 3.8 bar would be 2,850.345 mmHg.
So for the atmosphere to breathable at sea level, it would have to have a mixture of gases which are not toxic to humans at their indivdual partial pressures and which add up to about 2,400.1975 mmHg to 2,850.345 mmHg.
In Habitable Planets for Man, 1964, pages 13 to 19, Stephen H. Dole dicusses the atmospheric requirements for a planet to be habitable for humans.
On page 19 Dole writes:
To summarize, then, the atmosphere of a habitable planet must contain oxygen with an inspired partial pressure of between 60 and 400 millimeters of mercury and carbon dioxide with a partial pressure roughly between 0.05 and 7.0 millimeters of mercury. In addition, the partial pressures of the inert gases must be below certain specified limits and other toxic gases must notbe present in more than trace amounts. Some nitrogen must be present so that nitrogen in combined form can find its way into plants.
So a breathable atmosphere must contain betweeen about 60.05 and 407.00 millimeters of mercury (mmHg) of oxygen and carbon dioxide.
But a habitable planet will have liquid surface water, and thus it will have varying amounts of water vapor in the atmosphere. Dole discusses the humidity of a breathable atmosphere on pages 19 to 20. Table 4 on page 21 gives no minimum value for water vapor but gives 25 mmHg as the maximum. It also suggests that 10 mmHg might be the minimum pressure of nitrogen.
So a breathable atmosphere containing only the four essential gases would have a miniumum pressure of about 70.05 mmHg. Thus an extra 2,330.1475 to 2,780.295 mmHg of non toxic gas pressure would be needed to bring the total atmospheric pressure at sea level up to about 2,400.1975 mmHg to 2,850.345 mmHg while remaining breathable.
According to table 3 on page 16 and table 4 on page 21, up to 1,220 mmHg of argon would be breathable, and up to 3,900 (?) mmHg of neon would be breathable. Thus the maximum breathable pressure of argon and neon would total about 5,120 mmHg, 2,269.655 to 2,789.8525 mmHg more than needed. But the combined maximum tolerable leves of argon and neon would probably exceed human tolerance, so a much lower combined pressure would be desireable. It is is good that there over 2,000 mmHg to work with finding the maximum tolerable level.
But there would not be much need to mess around with neon and argon levels, since one of the essential gases, nitrogren, is tolerable at high pressures. Up to 2,330 mmHg.
So if there is a partial pressure of 2,330 mmHg of nitrogen at sea level, 0.175 to 450.295 mmHg of other gases like neon, argon, etc. would be necessary to bring the sea level atmospheric pressure up to about 2,400.1975 mmHg to 2,850.345 mmHg.
Assuming the atmosphere has the maximum amounts of the necessary gases, oxygen, carbon dioxide, and water vapor, that will be 432 mmHg. So to reach the required about 2,400.1975 mmHg to 2,850.345 mmHg, 1,968.1975 to 2,418.345 mmHg of various gases would be needed. Since nitrogen is breathable at partial pressues up to 2,330 mmHg, at most 88.1345 mmHg of neon, argon, xenon, & krypton would be necessary.
So if Earth humans colonize the highlands at 5,000 meters altitude, they should be able to survive breathing the air at sea level, if it has the proper mix of gases.
But only if the water vapor is low enough at sea level. The question gives the humidity at 90 percent at 5,000 meters and a much lower air pressure, and I don't kow how to translate that to mmHg of water vapor.
I note that at 50 degrees C, the specified teperature at 5,000 meters, 90 percent relative humidity would be 74.7 grams per cubic centimeter in one atmosphere pressure.
I guess it might be about 149.4 grams per cubic centimeter in two bar pressure.
More research would be necessary to say if the humidity would be tolerable to breathe at 5,000 meters above sea level, or at sea level.
Answers to this question contain formulas for calculating the scale heights of atmospheres.
And you might want to calculate the scale height of your atmopshere to see how dense it would be at sea level.
If the people of your planet are human colonists from Earth, they are likely to be quite unconfmortable living at 5,000 meters with air twice as thick as at sea level on Earth. And thus they will be even more uncomfortable with the ever thicker air as they descend toward sea level.
Of course a species of human like people who evolved at the high altitude of 5,000 eters should be quite comfortable breathing the air at that altitude. And when they descend lower to places with denser air, thatdenser air will be closer to what they are used to and and they were be more comfortable with it than Earth humans would be.