No, if it's a safety issue, you move to a full body lift. Also, just because policy states 1 person for the Sit to Stand doesn't mean you can't have 2 people for the transfer. Anyways, You notify the supervisor and document.
I.E "Resident cannot tolerate the sit to stand lift. Their legs give out and they fall back. Resident was transfered with a full body mechanical lift for safety."
Use the full body lift, to bed, to bedpan/bedside commode or shower chair for them to use the bathroom.
I don't see a problem with helping them get a hold of the handle bars and assisting with bending to sit.(Unless they can't hold on for the duration to complete toileting/transferring). The losing control and falling back is the issue.