Can i have two process networks using the same unmanaged switch?
At an ethernet level if you use one unmanaged switch you will only have one network.
At the IP level things get more complicated. Assuming that the two "networks" have non-overlapping subnets and there is no special routing configured then unicast traffic will not pass directly between the networks as the clients will see it is "out of network" and send it to the default gateway. The default gateway can then chose whether to pass it onwards to the other subnet or not (there is also an ICMP packet type that allows the gateway to tell clients to send future packets directly)
I'm not sure what will happen to packets sent to the subnet broadcast address, I expect machines in the other subnet will discard them but i'm not positive on that.
Packets sent to 255.255.255.255 will be received by all systems connected to the ethernet network. Unicast replies to them on the other hand will probablly only get through if there is a router available to do so (see above). This may confuse some systems.
A malicious device on one subnet could easilly detect that the other subnet exists and establish an address on it.
It's not uncommon to run multiple subnets on the same ethernet network due to address space constraints but you should not consider them isolated from a security perspective and you should be careful about anything that relies on broadcasts.