Her Majesty's Inspectorate of Constabulary in Scotland (HMICS) yesterday said the move should take place where necessary and called for more investment in often crumbling infrastructure inherited from the country's eight old forces.
Police in recent years have tried to make sure nobody they hold in custody shares a cell, but HMICS says this policy has to be eased.
In a review of the Police Scotland's new single Custody Division, the watchdog said: "The goal of single cell occupancy is welcome, but is currently creating capacity and resourcing issues that are unlikely to be resolved until there is significant investment in the custody estate."
It called for a "more proportionate approach to risk assessment allowing local discretion to use multiple cell occupancy where appropriate".
Police detained nearly 200,000 people in 2013-14. Historic figures are not available but the watchdog found anecdotal evidence that a new get-tough crackdown on domestic abuse means that more people accused of such crimes are being held.
Insiders say this has put pressure on custody suites despite an overall fall in crime.
Labour's Graeme Pearson MSP, a former senior officer, said: "This recommendation flows from a capacity problem from time to time when prisoners outnumber the cells available.
"Currently the policy of single occupancy creates a pressure to shift prisoners across the country to adhere to that approach.
"So long as Police Scotland provide custody officers with sufficient resources to properly monitor what goes on within the cell area, I can see little wrong with changing the approach.
"The problems arise, however, when trained custody staff are short-handed facing too many prisoners - some under the influence of various substances, particularly alcohol, some with mental illness and some violent in nature. That mix of people can be very challenging without the added complication of the multiple occupancy of cells. Tragedies in those circumstances are faced too often by the front line staff, and they need proper support."
Three people died in police custody last year. Two-thirds of those detained were found by the inspectorate to be vulnerable. However, it questioned the policy of "rousing" detainees every hour as they tried to sleep to ensure their safety, and found mixed levels of training across the eight former forces.
However, the HMICS also found that custody officers and staff were high in morale after being formed into their own division under Police Scotland.
Assistant Chief Constable Campbell Thomson said: "The delivery of Custody Division within Police Scotland has enabled us to develop a national model, which in partnership strives to care for detainees with fairness and respect. We welcome the HMICS report and its acknowledgement of our focus on continuous improvement and organisational learning.
"Police officers and police staff work in a challenging and complex environment, often dealing with highly vulnerable individuals who lead chaotic lifestyles. This report rightly recognises their professionalism, patience and genuine desire to care for those in custody.
"We are currently reviewing our arrangements for the provision of custody facilities across Scotland to ensure we continue to support local policing and collectively deliver an effective and efficient service."