It’s already split. In fact, it’s two votes, under two very different electoral systems.
One is a familiar, Westminster-style, ‘First Past The Post’ election for 73 constituencies, loosely based on the old, pre-2005 Westminster ones.
The other one is very different, a regional list election using the d’Hondt (or Additional Member System) method.
This means that at each stage of the count (and there are 7 Additional Members for each of the 8 regions) a party’s vote is divided by the number of seats it has already won in that region + 1.
Combining the two systems to elect one parliament is a fudge. It gives a result that in most cases is roughly proportional, without actually being a proportional system.
But it has glitches. One is that when a party does sufficiently well in the constituency section, as the SNP did in 2016, it can become all but mathematically impossible for them to win list seats.
In 2016, with an increased vote, the SNP gained 6 constituencies, and lost 12 list seats, leaving only 4,
Current polling at time of writing indicates the SNP could win 70 constituencies.
A million list votes could be cast for the SNP in 2021 and lead to not a single list seat being won.