Guitar AND bass strings on a bass (Royal Blood)

I was recently watching one of Royal Blood's live shows and I noticed on their song, 'One Trick Pony' that the bass Mike Kerr is using (a Gretsch short scale) has two bass strings and two, of what seem to be, guitar strings that he plays with a slide.

https://youtu.be/joHhmDpJXQI?t=17m00s

My Google searches haven't revealed much other than interviews where he mentions it briefly and another reddit post where someone posted a photo of a weird prototype bass.

https://www.reddit.com/r/Bass/comments/3jzsb7/interesting_new_bass_used_by_royal_blood/?

I know he plays with different octaves a lot but I though this was an interesting combination to point out and to see if anyone out there has done this before or has any more info.

submitted by /u/Hollahead
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Source: Bass