that would depend on your sanctioning body.

SCCA GCR is as follows
D. The single anti submarine strap of the five point system shall be attached to the floor structure and have a metal to metal connection
with the single release common to the seat belt and shoulder harness.E. The double leg straps of the six point or seven-point system may be attached to the floor as above for the five point system or be attached to the seat belt so that the driver sits on them, passing them up between his or her legs and attaching either to the single release common to the seat belt and shoulder harness or attaching to the shoulder harness straps. It is also permissible for the leg straps to be secured at a point common to the seat belt attachment to the structure, passing under the driver and up between his or her legs to the seat belt release or shoulder harness straps. All straps shall be free to run through intermediate loops or clamps/buckles.

While it looks like you might be able to get away with it, I assume it's not the best Idea.