Both ABS pressure values should be reading fairly close with the one upstairs being slightly lower ... about 0.3 mb less.
We need your elevation for the lowest sensor but a REL offset of of the same 4.5 mb suggest both sensors are at the same approx elevation of 37 or 38 meters?
If possible - might be easier to move both sensors side by side temporarily for easier calibration. You want both sensors to have the same ABS value if they are side by side.To do this, you adjust the ABS offset of each sensor until both ABS values are reading the same. Then move the GW1100 back upstairs and adjust REL offset using the higher elevation.
You can either calculate the REL offset for each GWxxxxby using the digital dutch standard atmosphere calculator or match the REL value with your source .
You may want to check this post for the procedure:
https://www.wxforum.net/index.php?topic=43477.msg442840#msg442840If after recalibrating, you find one sensor "drifting" then yes, there could be a problem.