How closely related are two parasites Giardia intestinalis and Trichomonas vaginalis?
Basically, Giarda is a an Excavates while Trichomonas is a Cercozoa meaning a Rhizaria (Chromealveolates in the broad sense as noticed by @har-wradim in the comments (now deleted) because Cercozoa are neither Chromista nor alveolata). Because the very basis of the tree of eukaryotes is still unresolved (eukaryotes), the most recent common ancestor of these two species is just an old eukaryote! In other words, these two species are not more closely related than are related a cat and a chestnut tree!

The links yield to Tolweb.org which shows you the phylogenetic tree and allows you to explore the relationships between organisms. Have a look to eukaryote tree especially to see the positions of the Rhizaria and Cercozoa.