Yes you can only use 1 exp scroll at a time.
EDIT:
Also if I remember correctly, I think once you're using a scroll and try to activate another it warns you that one is already in use , so it doesn't allow you to have more than one active at a time.
XP scrolls of different values do not stack, nor will the second scroll be consumed, if you currently have another XP scroll buff running.
Are you sure you're using a scroll that was 150 % and not psjic ambrosia at 50%? Also sounds like something you should take a snap shot of and contact support.
When you use an exp scroll, checking out your active buff will tell you which rate of %% you are currently using. So if you used a 50% scroll or Ambrosia, the icon will tell you you gain EXP at a rate of 50 % in all sources. Same with 150%. So if you used a 150% exp bonus scroll or potion, and your icon is stating you're only getting 50%, take a screen shot and contact support because that's not right.