What does it mean to get 2 phases of 110v outlet?
In the US, most household service is two legs of 120v plus a neutral. You can get 120v between either leg and neutral or 240v. Esteem both legs. In most breaker panels, single-width breakers alternate legs vertically and are opposite legs left and right per row. As you noted, you need to find outlets on breakers on opposite legs. 

For example, if the breaker slots are numbered 1-10 on the left and 11-20 on the right, you could use 1&amp;2, 2&amp;7, 3&amp;13 but not 1&amp;3 or 2&amp;11.

Also, these adapters are very dangerous. If you have one 120v plug connected and your 240v load connected but the other plug lose and you tough the prongs on it, you could get shocked. (These devices can be made safe using internal relays but most do not have these.)