Members of the Rail, Maritime and Transport union at South Western Railway (SWR) and Arriva Rail North (ARN) will walk out for 24 hours, with further action planned in the coming weeks.
Picket lines will be mounted outside stations across the two regions and some trains will be cancelled or replaced with buses.
ARN will operate fewer than a third of its normal Saturday services, while SWR said half of its trains will run.
The companies denied RMT claims that they had not responded to its most recent call for talks to try to break the deadlock.
RMT general secretary Mick Cash said: "It is a national scandal that while other train operators have been prepared to engage seriously with RMT on the crucial issue of the guard guarantee both ARN and SWR have dragged their heels and made a mockery of the talks process to the point now where they refuse point blank to engage in any meaningful negotiations at all.
"It is crystal clear that the axing of guards is a cash-driven exercise aimed at shoring up profits on these rail franchises regardless of the public consequences.
"Both ARN and SWR are known to be in financial trouble but passenger safety should not be the price the public are forced to pay for the on-going rail franchise chaos.
"We thank the public for their continuing support and understanding that these disputes are all about safety and passenger service on Britain's violent and dangerous railways."
An SWR spokesman said: "We have repeatedly guaranteed to roster a train guard on every single service we run. Our plans mean more guards, not fewer.
"It's time for the RMT union to commit to resolving their dispute which only causes misery for customers.
"We will do everything we can to keep customers moving and reduce disruption whenever the RMT strikes."
The ARN strike is the 14th consecutive Saturday walkout, leading to very few trains running before 9am and after 6pm.
Engineering work on parts of both operators will also see some disruption to services during the weekend.
Thousands of people are set to visit Christmas markets in Manchester and there are a number of sporting events across both regions affected by the strikes, including England's rugby game against Australia at Twickenham.
Last edited by karlypants on Sat Nov 24 2018, 09:59; edited 1 time in total