ULN2003 IC , should I stack it or should I divide the load?
You shouldn't use the ULN2003 at all, rather use a decent logic-level MOSFET. That's only 2.8A which is no problem for a MOSFET such as a IRLB4132PbF.
The Darlington switches in the ULN2003 drop a lot of voltage and thus get hot.
If you insist on using it, refer to figure 5 in the datasheet, each unit can only drive a single 400mA strip at 100% duty cycle. You should not parallel outputs that are not in the same package so you would really need 7 ULN2003, one for each strip.