What wavelength? I'm assuming 445nm. If so then why bother combining two 1.5W's? You would expect to see about 2.6-2.8W depending on losses. But instead you could use a single 3.5W+ NDB7A75 diode.
Also when combining it is important to note that you will have a '+' shaped beam, and reduced beam quality unless a waveplate is used.
Exactly.
PS there are no 1.5W Nichia 405nm diodes as others mentioned.1.5W Nichia 405nm x2 ,yep maybe it's good take a single diode like 6w+ with max peak 7.4W !
Even if there were---405nm is not a good choice if you want visual brightness---is OK if all you want is output/Watts power.
405nm is only 10% as bright visually as 445nm is .