Some printer drivers have this feature. I have a little Canon printer that can do this; just checked my Epson R800, that has the same facility. It would be worth checking your driver too. In both my cases, it was on the Page Layout tab.
Terry.

Ok. I see what you mean.
But overlapping produces a step in the surface that may look worse than a missing sliver.

There is a way to do it in Qimage from the batch filter but it means making however many images you require sections of the poster.
If, say it's 4 sections, make 4 separate crops of the image and save as separate images. When cropping in the batch filter, a read-out of the pixel size is given, just allow the appropriate overlap in pixels on the edges that will overlap.
I have actually done this in the past, not to make a poster, but just to test the technique, put them together on a page in Qimage and then printed to file to test the accuracy - perfect 
How about that?
Terry.

Thanks for your replies Terry.
My 'traditional' way of joining in to overlap on a light box 'tack' the back with masking tape and cut down the middle of the overlap. There's no step. No bit missing. No 'white' bits.
Of course it's possible to do it manually as I have been doing ..but it would be a nice option when poster printing.
Come on Mike make my day!! ..please

Seán