Use the short version of the 2Checkout signup form when checking out with PMPro.

2checkout_short_signup.php

/*
	Use the short version of the 2Checkout signup form when checking out.
	After installing and activating this, you'll also need to enable "SHORT_FORM" in your 2Checkout settings.
	See: https://knowledgecenter.2checkout.com/Documentation/05Ordering/10Minimize_required_checkout_data
*/
function my_pmpro_twocheckout_short_billing( $request_array ) {
	$short_form = array(
		"SHORT_FORM" => "1"
	);
	return array_merge( $short_form, $request_array );
}
add_filter('pmpro_twocheckout_return_url_parameters', 'my_pmpro_twocheckout_short_billing', 10, 1);