Yes, but sometimes not too advisable. In series that is. You CAN get to a situation where - if the motors are not precisely matched - one revs like crazy and the other idles away. NOT what you want on a twin...
For series, you need to just connect motors in series, then connect packs in series, and use a single speed controller rated at the normal current, but TWICE THE VOLTAGE,. These are hard tio get and expensive, and ofrten you will find you alos need a separate reciever battery, as teh BEC voltage is now exceeeded.
Its actually better to connect everything in parallel, and double up teh current capacity of teh ESC only. You can safely connect two packs in parallel, but don';t charge them like that.
However, the 'normal' think to do for twin 400's is to use 1700CP's or