Magento 2.1.2 Ui- Component formElement select from multiple dropdowns

Here's a way to do it by making a custom ui component. It requires the backing field to be a varchar.

Define the ui component in Your_Module/view/base/web/js/form/element/time.js:

define([
    'Magento_Ui/js/form/element/abstract'
], function (AbstractElement) {
    'use strict';

    return AbstractElement.extend({
        defaults: {
            elementTmpl: 'Your_Module/form/time'
        },

        initialize: function () {
            this._super();

            this.hours = '00';
            this.minutes = '00';

            this.observe(['hours', 'minutes']);

            var value = this.value();

            this.hours(value.slice(0,2));
            this.minutes(value.slice(2));
        },

        userChanges: function () {
            this._super();

            this.value(this.hours() + this.minutes());
        },

        hoursOpts: (function () {
            var opts = [];

            for (var i=0; i<24; i++) {
                opts.push({
                    label: i.toString(),
                    value: ('0' + i).slice(-2)
                })
            }

            return opts;
        })(),

        minutesOpts: (function () {
            var opts = [];

            for (var i=0; i<60; i++) {
                opts.push({
                    label: ('0' + i).slice(-2),
                    value: ('0' + i).slice(-2)
                })
            }

            return opts;
        })()
    });
});

and the template, in Your_Module/view/base/web/template/form/time.html:

<select class="admin__control-select"
        data-bind="
        optgroup: hoursOpts,
        optionsValue: 'value',
        optionsText: 'label',
        value: hours,
        event: {change: userChanges}"/>

<select class="admin__control-select"
        data-bind="
        optgroup: minutesOpts,
        optionsValue: 'value',
        optionsText: 'label',
        value: minutes,
        event: {change: userChanges}"/>

Use it in your form xml like this:

<field name="start_date">
        <argument name="data" xsi:type="array">
             <item name="config" xsi:type="array">
                <item name="label" xsi:type="string" translate="true">Monday Opening Time</item>
                <item name="component" xsi:type="string">Your_Module/js/form/element/time</item>
                <item name="visible" xsi:type="boolean">true</item>
                <item name="dataType" xsi:type="string">text</item>
                <item name="formElement" xsi:type="string">input</item>
                <item name="source" xsi:type="string">item</item>
                <item name="dataScope" xsi:type="string">start_date</item>
                <item name="sortOrder" xsi:type="number">220</item>
                <item name="validation" xsi:type="array">
                <item name="required-entry" xsi:type="boolean">true</item>
            </item>
          </item>
        </argument>      
    </field>

The value from the two drop downs is merged to produce a string like '0130' to represent the time 1:30, so your data type needs to be varchar otherwise the leading '0' will be dropped.