Beware that such a mutual fund
buy transaction closes at end of day. A sale of a stock or ETF closes 2-3 days in the future. So if you place both orders concurrently in your margin account, you may find you've borrowed from Schwab for those 2-3 days
on margin, at a rate of ~10%. Schwab's order screen will not warn you of this in advance.
What happens if you make this mistake in a "cash account" i.e. no margin allowed I do not know and hope not to find out by means of usual accidental carelessness.
An alternative to SWVXX is VUSB, which trades with standard ETF timing.