E coli cotransformation efficiency
There is no problem to transform bacteria with more than one plasmid. The usual methods (either chemical competent cells or electroporation) are working fine, although electroporation might give you better results, as the cells take up more DNA. On the downside you have to make sure that your cells are prepared nicely, otherwise the electroporation cell will produce a short circuit (of course chemically competent cells can be problematic, too).

What you will definitely need are two (or more) different selection markers. The easiest will probably be different antibiotics (something I used in the lab and which works nicely) or other like genes which provide essential substances for the cells. As long as the selection pressure on the cells is held up, both plasmids will stay in the cells, when you take away one of them, the corresponding plasmid will get lost within a few days.

Regarding the efficiency of co-transformations I haven't found any papers. There is however, some data available on factors which influence the effiency of single transformations and I don't see, why these shouldn't be applied for co-transformations as well.

First, there is the size of the plasmid. The bigger the plasmid is, the lower gets the transformation efficiency. The figure from Inoue, 1990 is pretty clear in this point:



So for this reason I would use plasmids as small as possible.

Second, the density at which the growing bacterial culture which is used to prepare the competent cells is harvested has a big influence on the transformation efficiency. Tang and colleagues (1994) found out that growing the cells to a higher density than the classical OD$_{600}$ of 0.6 (which for example Inoue recommends in it paper cited above) is not a very good choice in terms of transformation efficiency. They found an OD$_{600}$ of about 0.94 working much better, see the corresponding figure:



And finally the amount of competing DNA is a big factor. Hanahan, 1983 found out that the transformation efficiency gets lower, the more competing DNA is present:



So I would try to use as little of each DNA to work against this effect.