Connect two Ac to Dc adaptors in series
This will probably work OK provided that the outputs are both isolated.
 ie if both outputs have one side, say -ve, connected to input ground, then they cannot be combined in series. This would be unusual.  In most cases the outputs will be fully floating for DC relative to the inputs.

Some supplies will have a capacitor between output (usually negative) and input ground. Usually in the 0.01 - 0.1 uF range. This MAY produce some interesting noise results for circuits that cared but should cause no problem when used as you propose.

Connection is A-ve to B+ve
A+ve = 10 VDC +ve  out.
 B-ve = 10V DC-ve out 

In very rare cases it is conceivable that two switching regulators might go into some form of interactive oscillatory mode, but this would be extremely rare if it happened at all.