I grew up on a 5 + 4 Mack Quadruplex. Freightrain is correct about one thing; save the "split-shifting" with 2 hands till you get used to it! But, you don't necessarily shift the MAIN first. UPSHIFT whichever box you need to shift first, then DOWNSHIFT the other. If you try to downshift the main first, for example, you probably won't have enough RPM to do so. And, if you can, there's really no need to split-shift.
I can give you a couple of different patterns to use. There is also a slight difference between an 18-speed and a 20-speed. Since the Hi-Split and 5th geear are both overdrive in the 18, there are a couple of gear combinations which are duplicates, and should be skipped. On the 20, there are no duplicates.
First of all, the LO-LO on the compound is rarely used. In fact, the only time I ever used it, other than for backing, was to get moving in a steep place, and then only with one or two gears.
First Sequence (18-speed box) (this will work for most conditions, and has no split shifting):
DIRECT 4th (this is your 1:1 gear)
HI-SPLIT 4th (overdrive)
HI-SPLIT 5th (double overdrive)
Second Sequence (18-speed box) (for heavy loads):
If you need more on the bottom end, start out in LO-LO 1st, go to LO-LO 2nd, then to LO-SPLIT 1st.
On the 20-speed, just split 5th the same as 1st-4th.
The heavy-duty models had the 4-speed compound on the left, and the 5-speed main on the right. Some of the lighter ones had tat reversed.