Hi Phil
Looks like Thuja Occidentalis. Common to find as hedge or screening plants.
I dug a few of these some time back. One by one all are now mulch.
Branches refused to back bud and only grew at tips.
With time a couple grew shoots at branch crotches only .That means regrowing branches from scratch.
The twin trunk in your pic IMO is not worth the effort digging out. If the other has a better base have a go.
Three years ago I asked a more experienced bonsai grower. Should I collect 6 Thuja from my garden for bonsai.
His answer= " Mate you got nothing to loose and at least will get some experience in collecting".
After all the effort that's all I got, "collecting experience" which I could have done without.
However all experience collecting trees teaches something good I'm told
My thuja's were collected end of July in Sydney. One dug in March and did as well.
Hope that helps