On the IX board (for both the Menorah Kit and the Larson Scanner), there are 9 LED locations each of which is connected in series with a single resistor.
If you’re using red or yellow LEDs, each takes about 1.7 V, so each location could potentially take two LEDs in series. This doesn’t cost any additional current, but you might want to lower the resistor value to permit a full ~20 mA through each LED.
If you’re using other colors of LED (green, blue, white, UV, etc), then each takes ~3.6 V forward current, so you can’t run two in series.
You can also potentially put two LEDs in parallel. If you do so, you’ll need an independent resistor in series with each LED. You can remove (short across) the resistors on the board, and then take wire out from the LED location, to two LEDs, each in series with a resistor. The absolute maximum output per ATtiny2313 pin is 40 mA, so make sure that you don’t exceed 20 mA in each parallel branch.
(There’s a separate limit, of 200 mA, for total power on the microcontroller. If you’re using standard code, it doesn’t exceed 5 LEDs lit at a time; you’ll be OK.)
If you’re using red or yellow LEDs, you can even put two LEDs in series, twice in parallel, for a maximum of 4 LEDs per output pin.
See this article for additional information about resistors for LEDs in series and parallel: https://www.evilmadscientist.com/2012/resistors-for-leds/