From Flora of North America, which also accepts this split:

The taxonomic and nomenclatural treatment of Sanguisorba stipulata has varied. Plants from northwest North America and northeast Asia are not taxonomically distinguishable, a conclusion reached also by Li C. L. et al. (1994). The name S. sitchensis has been customarily applied to North American plants; the oldest name available for a taxon bridging the Bering Strait is S. stipulata. An additional taxonomic question is the relationship of this taxon to eastern North American S. canadensis. While similar and undoubtedly closely related, the two differ in several morphologic characteristics, are strongly allopatric in distribution, and are here regarded as sibling species.

In addition to geographic distribution, the following features can be used to distinguish S. canadensis and S. stipulata (from FNA):

• Leaflets oblong-ovate to lance-oblong, lengths ca. 2–4 times widths, bases broadly cuneate, truncate, or rounded to slightly cordate; calyx lobe midveins thickened distally; plants 9–20 dm......Sanguisorba canadensis
• Leaflets ovate-oblong to ovate or suborbiculate, lengths ca. 1.2–2 times widths, bases usually deeply cordate, rarely truncate; calyx lobe midveins not thickened distally; plants 2–12 dm......Sanguisorba stipulata

http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=129129