If your interface manual tells you that it shares some digital channels between ports like that (eg. SPDIF sharing ADAT channels 7,8)... well, that's that then. That's how they designed it and they're telling you just that.
Two steps to 'combine' multiple interfaces:
1. Combine multiple interfaces by making what's called an aggregate device with your OS audio utility app. (Audio MIDI Setup in OSX. ASIO in Windows)
2. Sync the sample rate clocks in multiple interfaces over word clock (if available) as the first choice. Over a digital audio connection as the 2nd choice. Over the data connection (USB, Firewire, Thunderbolt) as the last choice. Word clock is perfect stability and happiness and light. Digital audio connection usually works without error. (Might have trouble with HD sample rates over SPDIF though.) Data connection usually will give you troubles.
The more pro units let you connect as you wish and just work. (RME is pretty top end.
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Some of the budget USB models (especially if they both have no word clock and no digital ports to use) just weren't designed for robust full featured use like supporting aggregate devices.