If both are having Sankramana , it should be done on the first day. The Naimittika Shraddha is what is performed at the Solar and Lunar Eclipses and this involves Shad Devatas; and the Kaamya Shraddha is what is peformed out of volition and desire seeking a benefit like the Vratas on the Panchamis and in the Nakshatras like Krittika.Shraaddha Deshaas:Shraddhas are to be performed in the Dakshina Disha on a clean, cow dung washed, and without unclean material like worms, hair, bones and other doshas. Kathamchidyaadi Viprebhypona dattam bhojaney Madhu, Pindaastu naiva daatavyaah kadaachin -madhunaa bina/ ( In whatever way if honey is not offered in Brahmana Bhojana or on Pindas the offering is utterly inadequate). People who cannot afford should at least give a Sacred thread(Yagnopavitha). ShraaddhaNishiddha Desha-Kaalaas: Places occupied by Cows, Horses, Elephants and other animals or where Persons like Mlecchaas visit frequently are certainly prohibited to perform Shraaddhas. The Aavahana of the Pitru-Pitamaha-Prapitamahas is made: Amuka mamuka naama-gotra-Rupa mavayaahayaami; then one by one the Arghyas are given with the Mantras: Pitaridantey Arghyam, Pitaamahaadantey Arghyam, Prapitaamahaadantey : Swadhaanamah; Ya Divyaa/ The seshajala or the left out water is utilised for the Karta’s Mukhanjana or face-wash especially the eyes for Ayush Vriddhi. In case the death occurred in the night then munadana is stated to be due at the Deha Dahana itself. Sradha should be performed in the souramana month* , on the phase of the moon and on the thidhi when the person dies , annually. It is said that all the Pitru Karmaas upto Sapindikarana stand proof by the darbhas worn by the Karta as prescribed. Thus there would neither by an imagined Brahmana and need for Homa; since Brahmana Bhojana would be repeated again and again). Sradha should be done first according to rules Some people use grass instead of Dharbha and Barley instead of Gingelly. Can a Son in law attend and eat pithrusesham on the eve his mother in law's Sratham? The word 'resume' would be appropriate. For reading poems by Ramachander : http://poetrypoem.com/cgi-bin/index.pl?sitename=ramya475&;item=all 8.11He who has participated in a Sradha as Boktha cannot be a Boktha for another three days.
If the Karta were to have executed the Kumbha Shraaddha then he would reap the fruit of Ashwamedha Yagna.This Kumbha Shraaddha is to be done before Sapindeekarana in Ekoddhishta Vidhi followed by Paarana Vidhi.