I believe that every MTU can support a spyder that monitors 8 circuits. There are 4 numbered ports on both of the long sides of the MTU. These will accommodate 4 spyders on each side for a total of 8 spyders (circuits) on each MTU. So if you have 2 MTU's you can monitor 16 individual circuits. The Spyder tab will not show up in footprints until you activate and install the spyders.
I think that you are somewhat confused (or at least using the same term, 'MTU', for two different pieces of hardware in the same paragraph, which is liable seriously to confuse others).
TED uses the term 'MTU' for the small black boxes that each connect to a pair of CTs that (by design) monitor both sides of a primary circuit - usually either the utility feed or the PV solar feed. TED uses the term 'Spyder' for the similar sized black boxes that each have eight CT legs to monitor secondary circuits. Each
Pro MTU can support two Spyders, but the MTU only has one Spyder port, in addition to its main CT connectors. If you are using two Spyders on a single MTU, the second is 'daisy-chained' to the first. It is the Spyders that have four ports on each side, each of which can connect to a CT.
