Two
Medium Lasers will deliver 10 units of damage to a single area over the course of one full second at a distance of 270 meters or less, generate 8 units of heat, and will recycle every three seconds.
One
PPC will deliver 10 units of damage to a single area immediately upon impact at a distance of 90 to 540 meters, generate 9 units of heat, and will recycle every three seconds.
One
ER-PPC will deliver 10 units of damage to a single area immediately upon impact at a distance of 810 meters or less, generate 13 units of heat, and will recycle every three seconds.
The PPC delivers the same amount of damage at twice the range of the Medium Lasers, requires fewer hardpoints on a 'Mech, and has no concerns for "burn time" (making it better-suited for "off-the-hip" shots).
The ER-PPC has the same manner of advantages over the Medium Lasers as the standard PPC, while being able to reach
thrice as far
and being able to deliver that same 10 units of damage from barrel to range.
The primary advantages to using two Medium Lasers within their niche (instead of PPCs or ER-PPCs) is the lesser tonnage and (rather slight, in comparison to the standard PPC) lesser heat generation.
Lessening the heat too far on the PPC and ER-PPC (moreso with the former than the latter) removes one of the primary reasons to take Medium Lasers (among other things) in their stead, and is likely to result in close- and medium-range weapons being wholly ignored (or very nearly so) in favor of boated PPCs/ER-PPCs.
As noted above, twin Medium Lasers will produce 8 units of heat per salvo versus the PPC's current 9 units of heat per salvo.
They will deliver the same damage, for very nearly the same heat generation, at half of the range.
They will consume more hardpoints, and they don't even have the advantage of cycling more quickly.
Unless one has a rather tight tonnage or space budget or plans to constantly fight at ranges of less than 90 meters, why would one take two Medium Lasers over an equally cool-running (or potentially cooler-running) PPC?
That the proverbial "workhorse weapon of the Inner Sphere", among others, would become obsolete is part of why the PPC and ER-PPC heat levels are better off where they are.