How to set both CTRL keys to group switch?

Question

I'm trying to set up a primary US/DVP layout alongside an alternative APL/Dyalog one (to input APL characters) and I'd like to use the Caps Lock key (to the left of A) as Ctrl, and the two Ctrl keys as switches for the APL characters.

Basically what I'm trying to achieve is:

• Caps Lock → Ctrl ✅
• Both Shift together → toggle Caps Lock ✅
• Left Ctrl → Group Switch (APL characters) ❓
• Right Ctrl → Group Switch (APL characters) ✅
• Right Alt (AltGr) → Level 3 (for intl characters) ✅

I've almost done it, short of a Ctrl key:

rules:      evdev
model:      pc105
layout:     us,apl
variant:    dvp,dyalog
options:    caps:ctrl_modifier,shift:both_capslock_cancel,lv3:ralt_switch,grp:rctrl_switch

How can I set the original Left Ctrl key as a momentary group switch too, for APL characters, just like the Right Ctrl key? But without interfering with the new Ctrl key aka. old Caps Lock.