Using a DMM to measure output isolation/crosstalk
Info about DC/DC converters

DC to DC converters are usually isolating types with a rating of 1500VAC input to output. The rating comes from UL60950 and 1500VAC is twice incoming voltage plus 1000VAC. A lot of manufacturers of dc converters choose to meet this spec because, if the converter is an isolating type it's an easy design aim to achieve.

Back to your question

To want to test the isolation capabilites of the circuit board means there is some requirement for your company to do so and this may be because the power supplies are intended for medical or other sensitive applications.

You must therefore ask yourself what is the motivation for this test.

If the circuit board is designed correctly and parts bought from reputable sources then there should be no need to test isolation except whenever either this isolation is relied upon from a safety standpoint or the power supply outputs MUST be isolated to each other for correct funtionality.

If after all of this you decide it is a safety consideration, then you should do the proper test at 1500VAC (250VAC supplies) or equivalent peak converted to DC i.e. 2121Vdc.

If you decide it is a functional requirement rather than a safety requirement then using a DMM will barely indicate anything - try switching the incoming supply off and measuring resistance between one output and another - this will probably tell you more but still it is likely to be insufficient for just a functional requirement test.