Chris,
As proposed by Steve, those birds belong to the genus Progne. To be more exact the two individuals are Grey-breasted Martins (P. chalybea). Blue-and-White Swallow is all white below (incl. throat) and roughly the same size as White-winged Swallow (which also has a white throat). The individuals on the photo could perhaps be mistaken for Southern Rough-winged Swallow, but; Progne are larger, and their jizz is rather distinctive, exactly like the individuals on the photo. So, we are down to genus, then species: Brown-chested Martin has a whitish throat, so it often appears as if they have a chest-band. Thus it is eliminated. Female Purple is quite similar to Grey-breasted but usually show a lighter collar and forecrown. Also, the white belly/lower chest generally doesn't appear "clean" (often somewhat streaked) in female Purple. Caribbean Martin may occur very rarely in Guyana, but it is quite similar to Purple, and therefore also eliminated from the above. Accidentaly Southern Martin may occur in Guyana (there is a single record from coastal Suriname), but the underparts are never white in this species. Only possibilit left is Grey-breasted Martin, for which the birds are a perfect match :t: