It doesn't matter if Toon1 has an XP booster or not; the XP booster will only affect XP Toon1 earns, not the XP granted by the pact.
If Toon2 has an XP Booster, their XP will be increased by the appropriate amount, then the boosted XP will be split between the pact (Same goes for Patrol XP).
And Leveling Pact creation has been disabled since Freedom went live. Unless these are very old characters in a pre-existing pact, the answer won't matter.
@Roderick
On 2 accounts:
-Account 1-
PLer
Toon1
-Account 2-
Toon2
Toon1 and Toon2 have a leveling pact.
PLer PL Toon2.
Toon1 levels up whithout being online.
A) What if Toon1 and Toon2 have an xp booster?
B) What is Toon1 have booster and not Toon2?
C) What is Toon2 have booster and not Toon1?