What cut of ground beef is ideal to mix with ground lamb for kofta?
Try half and half sirloin and chuck for your beef. If you're grinding your own or can otherwise get quite specific, use sirloin tips. Ground, they're just as good as the much more expensive top sirloin. Sirloin will be low in fat, but will still have great flavor. I have used a combination of 1/2 lamb, 1/4 beef chuck and 1/4 beef sirloin tips to great success in Gyros meat (the meatloaf type).
Avoid round. There's a reason that round is often sold as generic stew-meat. It's a cheesy way for stores to sell what is often not very good meat. It's good for a few specific purposes, this isn't one of them.
EDIT: I just learned something I didn't know. Since I don't know what terminology is used in the Philippines, I bring you this handy diagram from Wiki.

I use the American terminology for sirloin, top sirloin, sirloin tip, round and chuck.
From Wiki:

The sirloin steak is a steak cut from the back of the animal.
In U.S. butchery, the steak is cut from the rear back portion of the animal, continuing off the short loin from which T-bone, porterhouse, and club steaks are cut.The sirloin is actually divided into several types of steak. The top sirloin is the most prized of these and is specifically marked for sale under that name. The bottom sirloin, which is less tender and much larger, is typically marked for sale simply as &quot;sirloin steak.&quot; The bottom sirloin in turn connects to the sirloin tip roast.