How to combine in phase 120VAC 15A 60 Hz feeds on a pair of outdoor GFCI outlets?
If you're in the US, what you're doing is dangerous, a code violation, and will not work.  If your equipment requires a 30 ampere circuit, you should install a proper 30 ampere circuit.

The National Electrical Code only allows circuit breakers to be connected in parallel, where they are factory assembled in parallel. Simply connecting two breakers in your panel in parallel, is a code violation.  There's also a section about not being allowed to connect conductors in parallel, unless the conductors are 1/0 AWG or larger.

National Electrical Code 2017
Chapter 2 Wiring and Protection
Article 240 Overcurrent Protection
240.8 Fuses or Circuit Breakers in Parallel. Fuses and circuit breakers shall be permitted to be connected in parallel where they are factory assembled in parallel and listed as a unit. Individual fuses, circuit breakers, or combinations thereof shall not be connected in parallel.
Chapter 3 Wiring Methods and Materials
Article 310 Conductors for General Wiring
310.10 Uses Permitted.
(H) Conductors in Parallel.
(1) General. Aluminum, copper-clad aluminum, or copper conductors, for each phase, polarity, neutral, or grounded circuit shall be permitted to be connected in parallel (electrically joined at both ends) only in sizes 1/0 AWG and larger where installed in accordance with 310.10(H)(2) through (H)(6).