A word for "every two days"

In regular conversation, the phrase is simply every other day. Technically, however, one could use bidiurnal. It appears the word may have been coined by Ursula M. Cowgill in her 1965 paper, A bidiurnal cycle in the feeding habit of Perodicticus potto, from which I quote thus (emphases mine):

A definite bidiurnal cycle exists; the data corrected for natural loss are shown in Figure 1. The χ² for the 48-hr periodicity is highly significant (χ² = 188).
_{Cowgill, PNAS, 420 (1965)}

The abbreviation QOD or QAD (from Latin mean Quaque Alternis Die") means 'every other day' or 'every two days'.

Modern style in medical situations recommends using using the spelled out English 'every other day' because the Latin abbreviations are often misread.

You could use "alternate".

I used to post on Instagram daily, but now I've switched to posting on alternate days. P.S Correct me if I'm wrong. ?

To solve your problem, though not to answer your question directly, for phrases of the type:

A definite bidiurnal cycle exists...

it is perfectly natural formal English to say

A definite two-day cycle exists...