You've shown the right hand capacitor connected to the wrong end of the right hand anode resistor.
I have to ask why you want to use valves (toobs) to produce squarewaves. If it's a gimmick, fine, but be aware that a squarewave from a toob multivibrator will not have "the toob sound". If that's what you're aiming for, you should probably think again.
It would also help if you would describe your project. I assume it's some kind of synthesiser, but more information would be appropriate.
The valve (toob) multivibrator does not produce a very clean output signal. Although the falling edge at each anode is steep and clean, the rising edge is not, because when the anode voltage is rising, the capacitor from that anode pulls the grid of the other toob positive, forward-biasing it relative to the cathode and making it draw current through the capacitor. This slows down the rising edge at the anode. This also happens in a transistor multivibrator. I'm just pointing it out, because if you want a clean squarewave, you'll need to add a clean-up stage, or change to a different type of oscillator.
Actually you might be best to use an oscillator with a single capacitor, whose frequency can be controlled by one circuit, feeding a bistable connected as a divide-by-two to give you an exact 50% duty cycle, if accurate duty cycle is important in this application.
The frequency of an astable multivibrator is determined by the time taken for the grid that has just gone negative (because the opposite triode has just started to conduct and has pulled its anode low) to rise to the point when the triode starts to conduct (which is slightly below 0V). This is determined by the current flowing into the capacitor. So the best way to control the frequency is to use a positive current source feeding into each grid.
The current source needs to be able to withstand the full HT voltage, because when the opposite triode has just turned ON, the grid goes negative by an amount roughly equal to the HT voltage.
I would imagine an op-amp-based circuit using a high-voltage PNP transistor. You might be able to use a single current source for both grids using diodes, because the grid that needs the current is always the most negative one.
Then you need to convert the control voltage from 1V per octave into a current that doubles for each octave. For example:
CV = 0V --> current = 25 µA
CV = 1V --> current = 50 µA
CV = 2V --> current = 100 µA
CV = 3V --> current = 200 µA
CV = 4V --> current = 400 µA
CV = 5V --> current = 800 µA
The formula for this conversion is: current = 25 µA multiplied by (2 to the power of CV). This is an exponential conversion and a circuit that implements it is called an exponential converter.
I Googled exponential converter circuit and found several designs, and lots of information about exponential converters in music applications:
http://electronicmusic.wikia.com/wiki/Exponential_converter
http://home.comcast.net/~ijfritz/sy_cir7.htm
