555 timer to control a servo with 2 variable positions
The diagram below shows an (untested) example of a circuit which may achieve your aims.
(The somewhat non standard 555 pin layout is the schematic editors idea.)
Apart from the 2 potentiometers and switching transistors operation is standard 555 astable.
The timing capacitor Ct charges via either Rchg1 or Rchg2 (see below) in series with Rdis until the capacitor voltage reaches 2/3 Vcc. The capacitor is then discharged via Rdis and the discharge pin until the capacitor voltage  falls to 1/3 Vcc.



simulate this circuit &ndash; Schematic created using CircuitLab
When the control input is low transistor Q1 is turned on via R5, and when required the capacitor can charge via Q1 Rchg1 and Rdis.
When the control input is high Q3 is turned on via R8, turning Q2 on via R7 which then allows Rchg2 to charge C1.
The charge times are thus settable via the two pots.
The discharge time is set in both modes by Rdis.
This has the effect of providing an invariant part cycle and a variable remainder. While this allows the mark-space ratio to be varied as desired it causes the frequency to alter with mark-space variation. Whether this is acceptable depends on your requirements.
While the discharge time could also be made selectable this requires more circuitry as Rdis is floating relative to ground and Vcc. While this is not technically hard the degree of complexity is unlikely to make it warranted.
A similar result could be obtained with an eg $US3 Arduino and NOTHING else if ratios are set in software. If variable ratios are required this could be achieved with two potentiometers or a few switches (eg up, down, channel select).
If an analog &quot;555 type&quot; solution is essential for some reason, using two astable circuits and switching between the outputs would require two ICs but less overall complexity.
if I had to do this with analog circuitry I'd probably use a package of CD40106 / 74HC14 hex Schmitt inverters or similar to implement two oscillators and switched output.