Old thread, but since I got here by googling, I figured I'd update this question.
Spud is correct, you can do ordinary LAG between the two, no need for MC-LAG (and those switches don't support it). New EX switches do support MC-LAG:
EX4300 Junos OS 13.2X51-D20
EX4600 Junos OS 13.2X51-D25
EX9200 Junos OS 12.3R2
Same link as hidayetkurtulmus mentioned is still valid (and updated):
/Fredrik