Hi Alex,
The difference between two boxes is that SRX240h ihas 1 Gig RAM , whereas SRX240H2 has 2 gig RAM.
As it is advisable from Juniper that hardware and software should be exact to form a cluster , combination of
these two models top form a cluster may result into abnormal behaviour , so i would suggest you not to use
these two devices as a cluster.
I have seen the setup working fine for a while , however may turn out to be a issue after months.
Regards
Hemant