Calculating power supply current output
Your converters should have a spec for efficiency.  Better yet, efficiency curves showing efficiency vs. output current.

Each of the 10V converters is putting out 1A, or 10W.  The input power is the output power divided by the efficiency at 1A.  

Suppose each converter is 90% efficient at 1A.  Then the input power is 11.11W for each converter.  Total input power is 22.22W.

Now the 15V supply has to supply 22.22W, meaning 1.48A.

Since these are DC-DC converters, output current doesn't necessarily equal input current like it would for an LDO.  The important thing to remember is that input power = output power/efficiency.