As Dexter Morgan has indicated, specifically not allowed in SG, but not mentioned in rifle so magazine coupling is GTG for rifle in IPSC.
And as tomo333 said IPSC and 3 gun are different beasts, so you'd have to look at the specific rule set of the match you are looking at as to whether they have any restrictions on coupled mags. Most matches it will be fine, but I have seen some rule sets that disallow it entirely, or disallow it if the mag bases are even so that you can't use a coupled mag as a bipod equivalent.
lonedrone