Hi Gaz,
There are SIBs that are well over 6 meters long and are rated for + 200 hp, so yes, you can tow 2 donuts with a SIB if it is big an powerful enough. For towing a single donut with a kid, you will probably need about 25 hp (with at least a 3.8 m long boat) for it to have any giddyup.
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