If you don’t remember to arrange two empty slots after the 50 km walk last week you can save it anyway.
1. Make sure that you incubate two eggs before 9 a.m.
2. Make sure that adventure sync is enabled.
3. Close the app and walk enough to be sure to hatch both eggs when you open the app next time.
Badaam! Eggs hatch before weekly reward is received and adventure eggs will fit.
Source: Did it today.
Left myself two egg slots, got two 5k eggs this morning. One hatched into a Buneary (under 82%), other one still has 1k to go.
Don't draw conclusions from a small sample size. Try this for two months - that should give you 8-9 Mondays, and you should get a few good eggs (and a few more crappy ones) from it.