A few cards can be locked to an external clock of some kind and share it (via special coax ports, or light-pipe ports, etc.) There’s also the option to just replace the ‘multiple-card’ layout with a single device that has the required number of inputs/outputs.
WINDOWS AUDIO LOOPBACK DRIVER DRIVERS
In the windows world, what would you recommend that’s better than ASIO? Cards do exist that can aggregate properly, and they tend ship with good drivers that hook into ASIO properly. If a device isn’t designed to slave to external clocks, or share clocks…then the ‘system’ is not going to work properly (the devices will get out of sync over time). So, it’s up to device makers to ship drivers with the features they intend to support. If every joe out there starts trying to aggregate audio cards, all on their own clocks, without understanding the potential pitfalls (and jumping through unsupported/unofficial hoops)…people will COMPLAIN that their outputs aren’t in sync and ‘drift apart’.
Cards that can do this ship with drivers that typically are limited to specific models or chip-sets that are designed to sync-up and work together. Technically, it’s against the protocol rules to aggregate multiple audio cards that are not designed to share a common clock, and ship with drivers and such to set it all up properly. Part of the reason they don’t open it up in terms of aggregating multiple devices, is the clock problem. Seriously, how can ASIO link dominate Steinberg at something they invented.