Not that it’s relevant in this case, because there aren’t any UCITS/NURS BTC ETFs yet, but in the UK one can only pay into a single S&S ISA (a tax-wrapper, the US equivalent of which I think would be a Roth IRA) in any given tax year, so it’s not always as simple as having multiple accounts.

Paying into a second account would require it to be outside the ISA scheme making the investments subject to CGT, etc.