Yes, you can.

When donors go to the marketplace and select the nonprofit they wish to donate to, they are able to use the 'advanced ' donation tab to access a slide bar and split the donation between the immediate donation and the sustainability fund in the way that resonates strongest with them.

Nonprofits can use a donation method that automatically allocates a ration between the two accounts. Some donation forms you may see on nonprofit pages may have 100% for immediate donation for example. That is their choice.
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On the marketplace pages, the default setting is 50-50.