The Rail, Maritime and Transport union (RMT) has banned overtime, but members will take extra action from 27 July.
The RMT said it plans to refuse to bring trains into service members believed were not safe. Aslef plans similar action but said it had been threatened with legal action over it.
London Underground (LU) said it would "never compromise" on safety.
Talks between union officials and LU have resumed at the conciliation service Acas to try to avert a second Tube strike planned for 5 August.
As part of further industrial action, the RMT said members would not bring trains into service if they did not believe they had been properly prepared and they would also not take part in any training or working practices not regarded as "normal".
In a message to RMT members, general secretary Mick Cash said: "We continue to try to resolve this dispute but LU has failed to properly address our claim.
"Their current offer is totally divisive and they have completely failed to address the wider issues of your work/life balance caused by their reckless efforts to bulldoze through the introduction of night Tube."
Members of the RMT, the Transport Salaried Staffs Association (TSSA), Unite and Aslef are unhappy about pay and shift arrangements for the new all-night Tube service, which is due to start on 12 September.
Finn Brennan, from Aslef, which represents some train drivers, said: "Rather than sitting down with us to resolve this industrial dispute, London Underground are threatening to take legal action to stop our members doing their jobs safely by making sure their trains are fully safety checked before entering service.
"It is now clear that their agenda is not to resolve this dispute but to force through changes, no matter what the impact on staff and the safety of their passengers."
The four unions previously walked out on 9 July causing the entire Tube network to shut down.
Steve Griffiths, LU's chief operating officer, said: "We will never compromise on safety and are operating the Underground safely for our customers and staff.
"Our measures to mitigate industrial action by union members include experienced managers undertaking some of their staff's duties.
"Any suggestion that we would use inexperienced staff is completely false."
A rail union is set to take extra industrial action in the row over the new all-night Tube service.