Luckily, the amount of cash savings required for ILR is less than for the FLR(M), so yes, if you were able to hold £19,500 in full for 6 months you could combine it with the USC's income of £15,100.
It's not really about sponsoring or self-sponsoring - once the USC is in the UK, their income counts as well... So you can either combine incomes or use whichever one meets the requirement.
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