my uncles is doing an addition to a rental and wants me to add another meter main to the existing service. i have yet to see the panel in person yet. can i add a double lug to the existing panel, so i can tap off the line side for the new panel that im adding?? i was trying to look for a 3gang horizontal meter main but couldnt find one online that would probably fit.

the pen markings in the pic is what i plan to do to make the new panel fit if i cant find a 3gang meter main

You can only double lug if the lugs and panel (your POCO too) are rated for such an installation. Both highly doubtfull!!!!

Not sure where the main is entering that panel? (Back or right side of the upper {metered} section? It could even be an under/under with no main OCP at all?) But interupting the main with a sealable gutter section, then tapping to each service would be my first thought. But the best and easiest bet would be to find a 3 gang meter/main, or a 3 gang meter and 3 discos. Either should be reletively easy to locate at a decent supply house.