Right. You may want to switch to Asiana''s program if it's better, but if not, just use one Star Alliance program for all 28 airlines. Unless you have some specific advanced frequent flyer inside baseball reason for multiple active programs in the same alliance, it's usually a dumb move to spread your earnings around.

Using the basic 25,000 mile domestic ticket (yeah I know bad value, it's an example!) as an example, and talking round-trips only:

12,500 miles credited to United gets you nothing.
12,500 miles credited to Asiana gets you nothing.

Those same 25,000 miles credited to ONLY United gets you a domestic US/Canada ticket on US/UA/AC
OR
Those same 25,000 miles credited to ONLY Asiana (probably) gets you that same domestic US/Canada ticket on US/UA/AC.

Now if getting status in the foreign *A program so that you get free lounge access at UA and US lounges in the USA when flying US-only itineraries is your goal, or getting some Asiana-members-only benefits when flying Asiana is your goal, while wanting similar UA-only benefits (like free E+) from United, and you fly enough to routinely hit top-level status in at least one program first before adding to the other program, sure, maybe it makes sense.

But for most people, you want one only *A program, only one SkyTeam program, and only one oneworld program. Or, only one *A program, plus Alaska Airlines Mileage Plan for a large subset of both SkyTeam and oneworld airline partners all in one "bank".

Having memberships in some mulitple without active miles earnings is useful sometimes such as for doing award searches if your "home" program has a lousy or nonexistent partner award online booking engine. Think UA before the CO merger, or AA now - The ANA, Air Canada, and Lufthansa sites were useful for looking for *A availability before calling United, and both the LANPass and BA Avios sites are valuable for finding AA award availability on oneworld partners of American. But earning across a mix of them is usually throwing away award earning opportunities.

Which reminds me - I really have to decide between using LANPass, or Alaska Airlines, or my new BA Avios (from BMI) program as my go-forward mostly-oneworld program. Already dumped United Mileage Plus in favor of AviancaTaca LifeMiles as my *A program.