I've done it; but without the proper relays it's dangerous; Electrical friend of mine calls it the 'widow maker'; You need to ensure L1 goes to L1; L1 to L2 is exciting...

Can be done. It is unpredictable and is not the safest in the world. It explicitly violates code and comes with insurance and liability risk.

In a perfect world, it'll work when the resistances of both paths are the same (same wire type, circuit length, and path back to the transformer). In practice, this isn't always the case: two pedestals may use different paths back to the distribution transformer, or even different distribution transformers. This can create a current imbalance that can cause overheating or overload of one of the parallel paths.

In addition, without safety interlocks, this type of paralleling can create exposed 240VAC on plugs that are expected to be dead when unplugged. In practice, this is similar to the "suicide cords" we see people use for generators, with two male ends. They're called "suicide cords" for a reason. In others, as nwdiver mentions, it can create a dead short if the ungrounded conductors are mismatched.

Finally, there are some older infrastructure installations out there that won't sync up at all. These are becoming rarer with time, but you may find older 240V delta 3-phase configurations that won't phase-align at all.

I don't recommend this practice.

In life my simple rule of thumb is - Risk vs. Reward. This potential risk is NOT worth the reward. MOST commercial locations now use 3 phase 208. You will see a nice fire if you combine l1 l2 or l3 together! 30MiHr really isn't that slow. There are superchargers everywhere I need but San Diego and SLC UT.