multi-interface i/o question

recently i built a live recording rig for a university job i have. i took a G4 and a tascam 1804 with DP4.6 and use a 16 or 24 channel mixer to either setup mics or split a FOH snake.

the G4 already has a PTLE setup in it with a card. i didnt remove that. i have been using DP for about three weeks and i am learning fast but still have a ways to go before i am an expert. recently i learned i can use both the digi card AND the tasam interface at the same time in DP. this is where my question lies.

to use both the digi card and tascam, i can select both in the hardware setup, they then show up on my track inputs. i have created a test session with this setup and routed sound into both interfaces and recorded it. this worked - no clicking or other clock issues between the two interfaces. i assume this is because i have my clock set to INTERNAL, and the clock is feeding both interfaces (?) i thought the digi cards used their own clock (?)

my question is this - on saturday i would like to use both interfaces - AND use an additional SPDIF input. i beleive i should set the clock in DP to SPDIF, but then do i need to run an additional SPDIF cable between the digi card and the tascam?

lets say i use the spdif input on the digi. i can set DP to recognize the digi as the clock source. does the tascam then follow this clock, or do i need to run a spdif cable from the digi spdif-out to the tascam spdif in?

i hope i made this clear. i am a little confused by this.