Shorting isn't the issue, heat build-up is, and bundles are usually considered to be 3 or more wires/cables. NEMA codes rate cables by size and insulation material and specify a maximum amperage for "free air" and a lower rating for bundled or enclosed cable. FAA and ABYC requirements are slightly different in their approach, ABYC has different ratings for "in an engine room" and "intermittent use" but they all attempt to provide standards which will prevent melting the insulation which then leads to massive shorts.
If you were to connect a #10 to your starter you'd probably melt the insulation and maybe the wire without turning over the engine, but two #1/0 cables tied together could probably handle simultaneous locked starters for a short period without a problem. Best approach is to use the charts and size and tie the cables as recommended.
Or then again you could consider running one cable to a terminal post and splitting it off to each starter, depending on how your starting banks are arranged. That probably won't work if you like starting both engines simultaneously, but most people don't.