What's the best phone app -

I use PicStitch. It's an older free app, its very basic. You do have to watch ads here and there to use it free, but just a few seconds.
When you first go in it shows the layouts in square format, but you can change the aspect ratio after you select the layout. So you would select 2 photos side by side, then select a 6x4 aspect ratio.

I primarily take, edit, & print my photos directly from my phone nowadays. I have some app suggestions and other techniques on my blog here:www.thepapergoddess.com/helpful-apps-photo-editing/

I don't think you can do it in PicStitch, or I haven't tried. When I print with the Canon app, I would have to choose borderless printing, which will bleed the photos on the edge, but there will still be a line down the middle. I have learned to live with the borders bcz it just was easier!

I have gone to other lengths to eliminate borders, like cropping the photos to the right aspect ratio then using a blank 4x6 layout I created in the Rhonna app. Loading that into the LetterGlow app and adding the photos & positioning them. I'm sure there are other ways to do it in other apps, but I try to just use what I have!

Like i said, I learned to live with the borders! 

I use picframe, I believe it's free. Same concept as picstitch. Pick 3x2 ratio and the frame with 2 vertical photos next to it. You can use a slider in the top right to change the corners to rounded or to make the frame bigger or smaller or even disappear. I use it all the time for my 3x4s! Message me if you want me to try to send you some screenshots. It's such a big help with my pictures, I'm happy to pass the tip along.