Footage of different framerates in Adobe Premiere Pro
It depends on the shutter speed being used on the other camera.  If the shutter speed is faster than 1/50th of a second, then shooting at 50fps and discarding every other frame would be the same as shooting at 25fps, but it means that your shutter speed must be faster than 1/50th of a second.

If light isn't an issue, then the faster frame rate won't have an impact, but if light may require a higher ISO and degrade the footage unnecessarily.  Similarly, if you aren't planning to need any slow motion shots, then there isn't really any great advantage to shooting at 50fps as you gain nothing.

Overall, say go with 1080p and 25fps on the GoPro unless you need to do slow motion and have sufficient light, in which case go 50fps on the second camera (or better yet, do 25fps for all but the slow motion shots and move to 60fps for the slow motion shots since the frame rate will be altered anyway.)