This seems odd at first, but it is completely understandable. If both amendments are passed, all that it does is put it right back where the bill is now. In other words, it would do away with the amendments and put back the language that was originally in the underlying bill.
Why would senators vote for both amendments then? Well, because many have learned their lesson. If they vote yes to one, and no to the other... those politicians will be politically smeared in the upcoming elections.
No matter which ammendment they voted 'no' on, the other political party will come out with advertisements that say "CAN YOU BELIEVE THAT SENATOR SO & SO VOTED 'NO' TO PRESERVING ENGLISH AS THE NATIONAL LANGUAGE." And since most Americans will believe this crap, it will hurt the politicians, even though they voted 'yes' to one of those ammendments that promotes English as our national language. In fact, this smear campaign happened on
these boards last time the ammendment was introduced.
So, in order to not be smeared, but yet vote for the ammendment they want, they vote for both. Right or wrong... that is why it has been done!
(BTW, most don't argue that English should not be our official language; they just disagree with whether the Fed or the individual states should make that decision. Colorado is an example).