I just activated my ATT phone for Straight Talk (ST) and ported my number from ATT to ST. So I have the same number on ST as I do on ATT. Obviously I am using the ST sim in my phone now. However, I still have my ATT goPhone sim and it has minutes on it for another 1-2 months or so. If I travel abroad, I won't have ST coverage in which case can I switch my sim to ATT in my cell phone?

I am using ST's data plan but never used ATT's data plan. So I had to add a new APN for ST data and was wondering if this could cause interference if I switch sims?
Yes, my phone is unlocked. It is GSM and could hence remove my ATT sim and replace it with the ST sim.