Hi Benjamin,
This problem is expected behavior for Active Active setup.
Add Static ARP at both the IRB interfaces then you should be able to ping both side:
set interfaces irb unit XX family inet address xxx.xxx.xxx.xx1/xx arp xxx.xxx.xxx.xx2 l2-interface ae0.0
set interfaces irb unit XX family inet address xxx.xxx.xxx.xx1/xx arp xxx.xxx.xxx.xx2 mac XX:XX:XX:XX:XX
Where the mac address is the one from the interface facing this irb.
Hope this helps.
Stefan Vesnic
Juniper Expert