Ian, this is a question to send to saturdays meeting. Under normal circumstances only one sling is allowed and it must remain attached to the rifle throughout the range. If you read rule 10.1b of the Main Shoot Rules it says -
i. Any shooting position is allowed, but some lanes may be designated as ‘standing’, or ‘kneeling’. In the event of a competitor not being able to conform to the above disciplines the following shall apply.
a. The competitor shall make it known to the Chief Marshal before the commencement of his/her round that they are unable, for whatever medical reason, to comply with a particular shooting position.
b. The Chief Marshal shall agree to the use of an alternative position or aid providing that no unfair advantage is gained from any special arrangements.
c. Other Marshals shall be made aware of any special arrangements.
I hope this helps