Bobbsy
Boring Old Git
I think you may find the choice of sync on your US 1800 is "Automatic" or "Internal". This comes into play when you use the S/PDIF digital input on the US 1800 to receive a digital feed from another A to D with a digital output. In this case, the US 1800 can sync itself to the incoming digital feed to ensure they remain in sync.
In this case, the combined inputs all appear to your computer as a single interface (i.e. the US 1800) via the USB 2 connection. It's a very different scenario than plugging in two completely separate USB interfaces and expecting both of them to work and remain in sync.
You make a good point though...a good way to work if you think you may occasionally need more than the normal set of inputs on your interface it to get one with S/PDIF or even ADAT (via TOSLINK) inputs. This will allow you to add more inputs later and still have them show up as a single interface to your computer and DAW. However, this takes planning when you buy your interface(s) and is unlikely to be possible at the economy 2 in/2 out interface end of the market.
In this case, the combined inputs all appear to your computer as a single interface (i.e. the US 1800) via the USB 2 connection. It's a very different scenario than plugging in two completely separate USB interfaces and expecting both of them to work and remain in sync.
You make a good point though...a good way to work if you think you may occasionally need more than the normal set of inputs on your interface it to get one with S/PDIF or even ADAT (via TOSLINK) inputs. This will allow you to add more inputs later and still have them show up as a single interface to your computer and DAW. However, this takes planning when you buy your interface(s) and is unlikely to be possible at the economy 2 in/2 out interface end of the market.