Switch mode supplies work by rapidly switching an inductive circuit fully on and off many times a second. Not sure offhand, but I think it’s generally in the 50,000~200,000 times a second range. Inductive circuits force current, not voltage. Thus the external load, as well as the built in following circuitry, becomes an important part of the voltage generated. Depends on the sophistication of the design, but in general switch modes are sensitive to reactive loads, inductance and capacitance.

Some of that switching works its way through to the output. Possibly the worst feature of switch mode supplies. If you connect 2 outputs directly, any small voltage mismatch, either average or switch noise, will drive a lot of current. The better the average voltage regulation, the better the quality of the supply if you will, the harder they will fight each other. Switch modes are typically well regulated baring any reactive issues, normally one of their best features. Switch noise might be less of an issue in a higher quality design however.

In regards to adding output capacitors, it depends a lot on the supply design. Again simpler switch mode designs are very sensitive to loaded capacitance and inductance. To be fair if your supply is like some of the laptop bricks I’ve seen, well they seem fairly heavy and large for a 5 amp supply, so maybe it’s a more sophisticated design, perhaps not so sensitive. I don’t know.

Bottom line, I’m not saying it absolutely won’t work, just reasons for caution. If you do try it please let me know how it worked out. Cheers!