Recently there has been a lot of press about a particular personality using 2 carriages in her knitting. This is not a new idea. Some points to ponder: if color changing is required there many ways to deal with it, beginning with doing it manually and devising a yarn holder of sorts to slip into space where the needle retainer bar sits. Then there are color changers, an absolute necessity in double bed work for DBJ. Not all machine models have changers that will work on both beds, Brother happens to be one that does not. Though Passap Autocolor will change colors automatically, the Brother single bed one is operated by one’s fingers pushing buttons, is a bit fussy, and it is really good not to hit an empty holder and go across with “no yarn”, since the object is usually not to have knitting fall off and onto floor.
Extra KH carriages are a bigger expense than color changers. Many production knitters have backups for their machine models. It is obviously best if both carriages are the same model year. Sometimes sequential model ones may be used, and all that may be required is a sinker plate adjustment, other times the carriages are incompatible with the new knitting beds though changes may appear to be small ones to the eye.
Unless I specify otherwise, my comments usually pertain to Brother KMs.
Aside from the fact that punchcard models have no power source, the pattern rotation is also different and that needs to be taken into consideration when punching holes in the card. Electronic machines advance a design row for each carriage pass on each side. Punchcard models do not.
In knitting stripes, the second carriage may house a thinner or thicker yarn, same yarn at a different tension, or hold the alternate color for frequent color changes. It may also be used with different cam settings than the other ie one for fair isle, the other for weaving or tuck, etc.
If combining stitch types a clear understanding of how punchcard holes and mylar or computer interface “squares” relate to needle selection and fabric formation is helpful and boils down to planning selection for needles one actually wants knitting. Patterning sequences must happen so each carriage makes an even number of passes, and returns home to its “side” for “automatic” use. Lace extension rails must be used and the alternate carriage is off the needle bed to avoid belt breakage.
The image below is a lo-tech “color changer” marketed decades ago. Old credit cards can be used for a DIY version.
Any circular knit that is shaped with sideways holding segments will need to be seamed when completed. Taking that into account, at least one row is knit with “doily” yarn from left to right after several rows are knit in waste-yarn before the transfers to create open stitches are begun. It is a personal choice and preference whether upon completion of the required number of segments to join by rehanging open stitches and binding off on the machine or grafting (the method I prefer) with the work off the machine after several rows of waste yarn and dropping the work off the KM.
Beginmomg with the LC on the left, the end needle selection is canceled. The two positions in LCs for both punchcard and electronic KMs
Back in 2002 exchanges with a fellow member of an Australian Yahoo Group, OzMKers, led to her final edit of this half punchcard repeat for the test doily. All transfers are in the same direction, to the right
A 24 stitch sample, knit in cotton yarn. The center of the circle needs to be managed as opposed to simply gathering it in order to keep the finished shape flat. 
It is possible for the lace carriage to transfer while at the same time preselecting every needle to be knit by the next pass of the knit carriage from the opposite side. An expanded view of part of the repeat: 
In order to get the repeat working properly, I found I needed to edit out one of the punched holes at the start of each repeat, revised card 
If drawing on the back of mylar for use on the 910, either image may be drawn as is, with the number 1 pattern case “A” selector reverse lever to up position.
In turn, this repeat as given was used in 2020 for a pattern test on my 930 for all transfers to the left. Note: here the knit stitch black pixels are followed by a blank row, and the transfer marks occur on design row 3, directly below the next series of all black pixels 
Depending on the machine model, the .png for the repeat may, in turn, need to be mirrored horizontally.
A 40-stitch adaptation from the Bond site to try
The electronic repeat as drawn on the mylar, 
Subsequent posts on knit edgings:
