Yes that's fine, the only issue you'll run into is that you can't configure the same VLAN in two separate vlan-groups on the MSFC side of things.
For example (assuming vlan20 is your DMZ):
SV4-10(config)#firewall vlan-group 2 20,30,35
SV4-10(config)#firewall vlan-group 3 20,40,45
Vlan 20 is already tied to group 2
So you need to create a separate vlan-group just for vlan20, then assign each firewall module to that vlan-group separately as follows:
firewall vlan-group 2 30,35
firewall vlan-group 3 40,45
firewall vlan-group 100 20
firewall module 7 vlan-group 2,100
firewall module 8 vlan-group 3,100
Other than that it should be fine to share the vlan amongst the modules. Just make sure that you PAT or NAT traffic going to vlan 20 from each module to a different IP address, that way the return traffic will know which of the two FWSM's to go back to.