In order to make my example clear, I'm going to make the following assumptions:
You're talking about the Layer 4 Traffice Monitor (aka L4TM)
Firewall inside interface, the WSA P1 interface and the WSA T1 interface plugged into the same swtich, on the same VLAN.
Firewall inside on gig1/1
WSA P1 on gig1/5
WSA T1 on gig1/10
(Optional WSA T2 on gig1/11)
If you feel that one interface on the WSA will get overloaded, set the L4TM config in Network/Interfaces for simplex. Then connect the WSA T2 port to the swtich (say gig1/11), and do an egress span from gig1/1 to gig1/10 and ingress span from gig1/1 to gig1/11.
If its all connected to a HUB, the use DUPLEX on the WSA L4TM config and just hook up T1...
Or you can use an actual tap, that you put in-line like this: http://www.netoptics.com/products/network-taps/101001000baset-tap and hook that up to the T1/T2 ports as appropriate.
Hope that helps!