The problem is, that whilst many English voters (me amongst them) would vote for an SNP candidate if they thought it would increase the possibility of Scottish independence, having the UK Parliament top heavy with representatives who have no regard for the English would not be a good idea.
Personally I think that any referendum on Scottish independence which is restricted to solely Scottish voters is unfortunately bound to fail – and fail heftily. They had their best chance a couple of years ago and got the closest they are likely to get to success for a generation or two. Their best chance is to persuade the UK government to extend the plebiscite to include all UK voters. But I don’t think too many English voters would tolerate seeing the Westminster forum stuffed with SNP MPs which would be needed to achieve that aim.