So, when I configure the HPNA card I should use 255.255.254.0?

No, you're still thinking the mask is the address. The mask is just a mask.

Just set the mask to 255.255.255.0 on both interfaces and leave it. Now forget it for now, pretend you never heard of the mask.

Now, set the actual dhcp server address ranges of the subnets served by the two cards as follows:

Existing ethernet address range:
192.168.0.xxx

New card address range:
192.168.1.xxx

Your addresses may be different, but the point is to set the HPNA card to a different subnet (in this case, one digit off) from the current subnet.