I had been using 6g of L-Citrulline for about four months with good but not spectacular results when I read about combining it with dietary nitrate from beet root juice. The idea is that citrulline works through the L-arginine/NOS pathway (making more NO from arginine) while dietary nitrate works through a completely independent pathway (nitrate is reduced to nitrite by oral bacteria, then to NO in acidic environments like the stomach and exercising muscle). Combining them hits nitric oxide production from two directions.
I added 500ml of beet juice (approximately 400mg of nitrate) about 2 hours before training, keeping my 6g citrulline at the 45-minute mark.
The difference was immediately apparent. The pump was at a level I had never experienced before from any supplement combination. During a high-volume arm session (20+ sets total for biceps and triceps), my arms were so engorged that I had difficulty fully extending my elbows between sets. Vascularity was extraordinary -- veins I did not know I had were visible through the skin.
More practically, I hit a 12-rep PR on barbell curls and added an extra set to every exercise because I simply was not reaching failure as quickly as expected. The endurance effect was more pronounced than citrulline alone.
The downsides: beet juice is not cheap if you buy it regularly. It turns your urine and stool red or pink, which is alarming if you are not expecting it (beeturia). And the combo made me slightly lightheaded during my first heavy squat session, which I attribute to the additive blood pressure-lowering effects. I now avoid the combo on heavy compound lift days and save it for isolation-heavy volume sessions.
This is not a combination I would recommend to someone new to supplements. Try citrulline alone first, establish your baseline response, and then consider adding dietary nitrate if you want to push the NO envelope further. But for anyone chasing the most extreme pump possible through legal supplementation, citrulline plus beet root is the gold standard in my experience.