You can basically add bacteria either 1-2 days after beginning of fermentation (early co-inoculation), or around 0 Brix (late co-inoculation), but adding bacteria in the middle of AF is the worst time. Late co-inoculation is a little safer because you can see if the fermentation curve looks good, and add bacteria. If the ferment is going to be sluggish, you should avoid adding ML bacteria then. Take care of yeasts first, before adding other microbes.
Download the Handbook for Co- Inoculation here.