Optimus is a catch-all term for the design of a laptop GPU to assist the intel integrated graphics, however the exact design strategy (and therefore usage ramifications) varies by laptop, as well as by OS. Newer OS (e.g. win10) have recognized this hybrid graphics scenario and have specific support features for it, for example.
Anyway, this means that an Optimus GPU may be essentially a stand-alone GPU that can be placed in TCC mode, or it can mean that its design is so intertwined (both from a HW and SW perspective) with the integrated graphics that it is really not a standalone GPU and cannot be placed separately into TCC mode.
If you have further questions about a specific laptop design and supported usage models, you should contact the manufacturer. NVIDIA doesn’t give advice on such configuration topics for every laptop.
The card needs to be in TCC mode to avoid the WDDM timeout. Even a GeForce card that is not driving a display still has a WDDM driver stack built on it by Windows (after all, you could go into windows control panel and enable a display on that card at any time). This WDDM driver stack may enforce card responsiveness (i.e. the WDDM watchdog) even if the card is “not being used” by windows.
Yes, if you have two GPUs in a system, and one of them is in TCC mode, then the TCC mode GPU should not be affected by the WDDM timeout.