I am currently working on preparing a 9 kb sequence of DNA for restriction digestion into the pBAD30 expression vector. There are very few restriction enzymes that do not have a restriction site located on my insert, and since I am using 2 restriction enzymes in my digestion, I had little choice in choosing my restriction enzymes. The only two restriction enzymes that will work for me are Xmal and KpnI. XmaI uses CutSmart buffer while KpnI uses NEB buffer. The efficiency of XmaI in CutSmart buffer is 100% while the efficiency of KpnI in CutSmart buffer is 50%. Would it be easier to perform a double digestion using CutSmart buffer and 2x KpnI than to perform two consecutive digestions?
In my experience, a double digestion in CutSmart buffer will work perfectly well. The reaction may proceed slower, but incubate it a little longer and run a gel after the digestion - you'll see whether it has worked. The other answerers unfortunately did not mention that the best way is to check the restriction products yourself on a gel. Cut out the fragment from the gel you need (and purify) if you want to avoid undigested plasmid. See if the plasmid fragment sizes correspond to the correct cutting sites. Basic lab practice and it takes very little time!
Beware of star activity - ask a technician to sequence your fragments or plasmids to be sure it's all going well (this is good lab practice), in case you see unexpected bands on your gel that may indicate unwanted restriction activity. This is not so common, though :)
NEB double digest planner is suggesting to use 2.1 buffer for your combination of restriction enzymes (XmaI 50% and KpnI 75% activity). I would be more worried about the star activity of KpnI in CutSmart buffer than the lower activity. The latter can be overcomed by prolonging reaction time or using more enzyme.
If you haven’t already ordered your enzyme, you could get high fidelity KpnI, which has been engineered for use in CutSmart.
You could also consider doing a sequential digest rather than a double digest (ie digest with one enzyme in its optimal buffer then change the buffer for digestion with the second enzyme). It is more work but benefits from easier troubleshooting in case one of your enzymes is not cutting (which can occur frequently if you have a freezer full of 10 year old enzymes).