Steel sheets can absolutley be spliced together, whether in a low headroom application or not. IIRC, there are even splice guides, similar to the ones used for H-Pile splicing.
I would caution though, that the integrity of the spliced sheet as a moment resisting element may not be as reliable as the solid sheet was. Take that into consideration in your calculations.
If you splice the sheets, make sure that the upper and lower pieces are aligned properly. You could have difficulty driving the sheets if the spliced interlocks are not straight.
?I vaguely recall doing one decades ago where I welded a lug on the side of the sheet, drove that to grade then set the vibro on top of the sheet to finish it off. Might save splicing.
PSlem, I've also been thinking about it and you might be onto something (not "on something"). Whether sheet piles or soldier piles we need to know:
1. design depth of fully driven sheet piles
2. length of sheet piles being used
3. headroom available
4. height needed to accommodate driving equipment (is it a vibro?)
It may be possible as suggested by PSlem to partially drive the sheet piles by having a scondary lug, or even if using a backhoe mounted vibro to pull the piles down until there is sufficient headroom to drive from the top of the pile.
