Technically, he could endorse both Trump and Rubio. Not likely or probable, but it’s a choice that makes use of the word “either” imprecise and arguably misleading. Just sayin’
If Cruz endorces Rubio, Trump will go up at least 3-5% in the polls, as the folks who backed Cruz thinking he was a principled conservative, and anti amnesty, see him sell out... those folks will either stay home or go Trump.
If Rubio wants maximum a benefit from Cruz exist, the better path is probably for Cruz to endorce no one I would think.
Time will tell.