Not quite sure I understand the question. The DEXT16 only controls the DSSR20 so DEXT16 is connected to the small grey connector to Batt and Heat and when charging Batt is enabled and if not charging the Heat will be enabled.
Then on DSSR20 when changing of Lithium battery is not needed the Heat+ will just output the PV panels so is like a switch that diverts the PV panels to Heat+ output and so if that PWM charge controller can handle the panels connected to DSSR20 then yes you can connect that PWM charger PV input to the Heat+ so that you can maybe charge a Lead Acid battery when the main Lithium battery is fully charged.
Is that a 12V/24V PWM or just 12V as it seems there are multiple variants and what panels do you have ? If you have 60 cell panels and PWM controller is 12V/24V then it should be fine but the charge will be limited to 20A so half of the panel's power will be utilized. If is just 12V then you need to check the spec to see what is the max PV input voltage for that PWM charger.