But, mousey, it's not a split infinitive at all! 'to someone tie up' would be. But this is a compound verb, (to tie + up), 'to tie' being the infinitive, and 'up' being an additional part of speech whose name I can no longer remember; it is correct to put the object (in this case, the thing - or person - being tied. Up.) between the infinitive 'to tie' and the - whatever, may be a substantive - 'up'