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I have found a solution and answer to my own question:

Registering a listener in wp.data
query the currently selected block in the listener function
compare the ID of the currently selected block with that of the previously selected block
according to the result, set the current block to 'edit' and the other to 'preview' mode

To avoid an infinitive loop, I used the throttle function during registration.

 function makeListener() {

    let prevBlockId;
    return function updateBlockMode() {         
        const currentSelectedBlock = wp.data.select('core/block-editor').getSelectedBlock();
        const currentBlockId = (currentSelectedBlock) ? currentSelectedBlock.clientId : null;

        if (currentBlockId === prevBlockId) {
            wp.data.dispatch('core/block-editor').updateBlockAttributes(currentBlockId, { mode: 'edit' });
        } else {
           wp.data.dispatch('core/block-editor').updateBlockAttributes(prevBlockId, { mode: 'preview' });
        }       
        prevBlockId = currentBlockId;
    };
 };
const unsubscribe = wp.data.subscribe(_.throttle((makeListener()), 200));

This approach works for me but maybe there is a better solution. I would appreciate any ideas and suggestions.

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