Digitizing traditional art

[mekare]

Hey, I hope some of the graphics programme users here can help me. I have finished a big watercolour piece today and tried to scan it in two parts. Now my idea was to merge those two parts in a graphics programme. Has anyone ever done it successfully? I only own a freeware programme (GIMP) which is supposed to be similar to Photoshop and there was definite line separating the two scans. Any idea on how to get rid of it or a better method? I tried taking photos of the whole picture but they came out much darker than the scan (I adjusted brightness and contrast of the photo in that same programme but it's still not as light as the scan).

ETA: Thank you everyone for all the helpful tips. I'll definitely try the overlaying this weekend with an additional middle part scan. I knew I could count on you :-)

ETA2: I think I did it! I put two scans together and adjusted the brightness of the lighter one, scaled down the resolution somewhat and now the line is almost unnoticeable. Thank you all! Unfortunately I can't post the result yet, since it is a Bigbang piece, but I'll post a link when I'm allowed.