I need around 20amps to run my mobile HF rig in the house. I have lots
of old PC switching power supplies that put out 8 - 10 amp @ 12v
without any problem.
Im wondering if anyone has ever had any luck connecting two or more
in parallel to increase current ? Ive thought of adding a stud
mount diode in the positive leads to isolate them and connect the
common or neg together. I can raise the voltage to makeup for the .6
volt drop without any problem.
Not having a lot of switching supply experience, I wonder if this
will work or just go up in smoke?

I wouldn't recommend it. There are computer power supplies made for
N+1 hot swapping that will work well in parallel, but they are designed
for that application. Look around for a surplus power supply big enough
to handle the radio.
======================
Another consideration is to use a 12 V lead acid battery (sealed) and a say
8 Amperes charger set at 13.8 V. being 1 PC -SMPS

The peak current required for 100 Watts RF operation might be 20 Amperes
,but the average current for SSB during tx is below 8 Amperes ,even when
applying compression. With the current required during rx around 2 Amperes
,the overall average current will probably not exceed 6 Amperes.
With this setup the voltage will hardly drop below 13 volts ever.

An additional advantage of having such a supply system is emergency
operation during an AC power outage. In the latter case you should consider
a large marine type of battery of say 110Ah.
If prolonged operation during an emergency is an important aspect , the
addition of 1 or more solar panels would be beneficial if not essential.

Frank GM0CSZ / KN6WH