The Bhutanese 14-month-old twins, who are joined at the torso, are reported as well and in a stable condition after being admitted to Melbourne's Royal Children's Hospital on Friday.

But both girls will require nutritional support, physiotherapy and other health support before doctors can proceed with the delicate surgery in coming weeks, the hospital says.

Nutrition levels will be an important guide, with the girls' condition meaning they cannot be weighed separately.

The girls had a four-hour MRI scan on Thursday and the results will be reviewed before doctors confirm their treatment plan early next week.

They ate dinner and stayed overnight at the Children First Foundation's retreat in Kilmore.

"They're pretty skinny, they haven't been able to practise crawling the way normal kids would, so maybe their muscles haven't developed so well but we want to just double check everything is alright, generally," lead anaesthetist Ian McKenzie said on Thursday.

The planned separation includes splitting the girls' shared liver.

The hospital's head paediatric surgeon Joe Crameri said image scans will be key to determining the best time to separate the twins.

The surgery and recovery are estimated to cost at least $350,000.

The state government has pledged to cover the surgery costs, with other funds raised to go towards the girls' stay, rehabilitation and future.

Australian Associated Press