Multiple, repeating modifier keys in Karabiner

I am in the process of setting up Karabiner to replace ControllerMate for an N52 Nostromo controller, which now shows up as the left-hand portion of a standard keyboard. I have a number of complex modification rules, all of which are using "frontmost_application_if", and "device_if" to limit scope. I have two rules that work on their own, but do not work when pressed together:

"from": { "key_code": "r" },"to": [ {"key_code": "left_shift", "repeat": true } ]

"from": { "key_code": "spacebar" }, "to": [ {"key_code": "left_alt" } ]

With 'R' held, I get 'shift', but once I also press 'spacebar', instead of getting 'alt', I get 'shift' and 'spacebar' I also tried setting a variable with the shift rule, and had a second 'alt-shift' rule that triggers on 'spacebar' + the variable, but it won't fire, and it seems like it has something do to with a modifier being held on repeat. Can these be combined?