You can do it in LR Eport, but only with a third party plugin called LR Mogrify - a very handy donationware plugin from the Photographer's Toolbox
(Aside: Adobe should pay a substantial amount to TPT to acquire LR Mogrify and build into LR!)
It does a lot more than you have requested, but both images could be exported at the same time by setting "Long Edge" to 1500px in the Image Sizing pane, then, in the LR Mogrify "Background Canvas" pane, specifying a 1500 x 1500px canvas.
You can see LR Mogrify at work in a recent post of mine
Hope this helps.