Given that most people don't have an HSA worth more than a few percent of their overall net worth, I personally think that putting it in more than one fund just tends to unnecessarily complicate your financial life. Pick one fund such as a US total market fund (or S&P 500 fund if total market isn't available), and put the whole thing in there. Adjust your other accounts as appropriate to make sure that your total portfolio matches up with your desired asset allocation.
^^ This is my approach, and I have Health Equity for a HSA too. I dump it all into VIIIX -- it's the best option available to me in my Health Equity plan.
My plan lets me invest anything above $2,000.00 (must keep a minimum $2K in cash), but I get charged a monthly fee if I do not keep a minimum $2,500 in cash. So, I invest everything above $2,520. I keep a small buffer to make sure that I meet that minimum $2,500 -- the monthly fees for the investing portion of the HSA get deducted from the cash portion, and I don't want to dip down below $2,500 cash.